NetFilter/IPTables

Dear All,

Hope you guys are doing good! I am in need of a stand-alone versions
of portions of NetFilter/IpTables. What I really want is a
user-application-level-C-program that does 'computationally' something
close to what various parts of NetFilter/IPTables do. So it needs to
be free from the networking stack and all other kernel-related issues
- since it's in userland. I just want a plain simple Program that
emulates closely what NetFilter/IPTables does, and I am basically
looking for Network Address Translation (NAT), Routing, and also
packet filtering.
I am thinking of snipping off portions of the netfilter/iptables code
that corresponds to each of those parts. Can anyone suggest which
c-files to look for code corresponding to NAT, routing, and Packet
Filtering? Any other suggestions are welcome too.

The stand-alone program will not actually be dropping packets as
such...but will be just reading in a text file or some trace for
packets and then performing lookup for routing, NAT and filtering on
that packet.

Cheers!
Learner